Silicon particles for battery electrodes

ABSTRACT

Silicon particles for use in an electrode in an electrochemical cell are provided. The silicon particles can have surfaces providing an average contact angle less than about 87.2°. The silicon particles can also have outer regions extending about 20 nm deep from the surfaces. The outer regions can include an amount of aluminum such that a bulk measurement of the aluminum comprises at least about 0.01% by weight of the silicon particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/596,071, filed Dec. 7, 2017. The entirety of the above referencedapplication is hereby incorporated by reference.

BACKGROUND Field

The present application relates generally to silicon particles. Inparticular, the present application relates to silicon particles andcomposite materials including silicon particles for use in batteryelectrodes.

Description of the Related Art

A lithium ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of batteries, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. Typical electrodesinclude electro-chemically active material layers on electricallyconductive metals (e.g., aluminum and copper). Films can be rolled orcut into pieces which are then layered into stacks. The stacks are ofalternating electro-chemically active materials with the separatorbetween them.

SUMMARY

In certain embodiments, silicon particles are provided. The siliconparticles can be used in an electrode in an electrochemical cell. Thesilicon particles can have surfaces providing an average contact angleless than about 87.2°. For example, the average contact angle can befrom about 82° to about 87.1°. The silicon particles can also have outerregions extending about 20 nm deep from the surfaces. The outer regionscan include an amount of aluminum such that a bulk measurement of thealuminum comprises at least about 0.01% by weight of the siliconparticles. For example, the bulk measurement of the aluminum cancomprise from about 0.01% to about 1% by weight of the siliconparticles. In some instances, the bulk measurement of the aluminum cancomprise at least about 0.05% by weight of the silicon particles. Forexample, the bulk measurement of the aluminum can comprise from about0.05% to about 1% by weight of the silicon particles. In some instances,the bulk measurement of the aluminum can comprise at least about 0.1% byweight of the silicon particles. For example, the bulk measurement ofthe aluminum can comprise from about 0.1% to about 1% by weight of thesilicon particles. As another example, the bulk measurement of thealuminum can comprise from about 0.1% to about 0.6% by weight of thesilicon particles. The outer regions in some silicon particles cancomprise aluminum oxide and/or aluminum silicide. In certainembodiments, electrodes for use in an electrochemical cell comprisingvarious embodiments of the silicon particles are also provided.

In various embodiments, composite material films are provided. Thecomposite material film can include greater than 0% and less than about99% by weight of silicon particles. For example, the composite materialfilm can include the silicon particles at 50% to 99% by weight of thecomposite material film. As another example, the composite material filmcan include the silicon particles at 70% to 99% by weight of thecomposite material film. The silicon particles can have surface coatingscomprising silicon carbide or a mixture of carbon and silicon carbide.The silicon particles can have regions extending about 20 nm deep fromthe surface coatings. The regions can include an amount of aluminum suchthat a bulk measurement of the aluminum comprises at least about 0.01%by weight of the silicon particles. In some instances, the bulkmeasurement of the aluminum can comprise at least about 0.1% by weightof the silicon particles. For example, the bulk measurement of thealuminum can comprise from about 0.1% to about 1% by weight of thesilicon particles. As another example, the bulk measurement of thealuminum can comprise from about 0.1% to about 0.6% by weight of thesilicon particles. The regions in some silicon particles can comprisealuminum oxide and/or aluminum silicide. The composite material film canalso include greater than 0% and less than about 90% by weight of one ormore types of carbon phases. At least one of the one or more types ofcarbon phases can be a substantially continuous phase. In someinstances, at least one of the one or more types of carbon phases thatis a substantially continuous phase can be electrochemically active andelectrically conductive.

In some embodiments of the composite material film, an average particlesize of the silicon particles can be from about 0.1 μm to about 40 μm.For example, an average particle size of the silicon particles can befrom about 1 μm to about 20 μm. In some instances, the silicon particlescan be from about 90% pure silicon to about 99% pure silicon. Thesurface coatings can include silicon monoxide (SiO), silicon dioxide(SiO2), or silicon oxide (SiO_(x)). The surface coatings can be asubstantially continuous layer. The composite material film can beself-supported. In various embodiments, lithium-ion battery electrodescomprising the composite material film are also provided.

In certain embodiments, methods of forming a composite material areprovided. A method can include providing a mixture comprising aprecursor and silicon particles. The method can also include pyrolyzingthe precursor to convert the precursor into one or more types of carbonphases. The method can further include forming silicon carbide onsurfaces of silicon particles. The silicon particles can have regionsextending about 20 nm deep from the surfaces. The regions can include anamount of aluminum such that a bulk measurement of the aluminumcomprises at least about 0.01% by weight of the silicon particles. Insome instances, the bulk measurement of the aluminum can comprise atleast about 0.1% by weight of the silicon particles. For example, thebulk measurement of the aluminum can comprise from about 0.1% to about1% by weight of the silicon particles. As another example, the bulkmeasurement of the aluminum can comprise from about 0.1% to about 0.6%by weight of the silicon particles. In some instances, the siliconparticles can comprise aluminum oxide and/or aluminum silicide.

In some embodiments of the method, the surfaces of the silicon particlescan provide an average contact angle less than about 87.2°. For example,the average contact angle can be from about 82° to about 87.1°. In someinstances of the method, providing the mixture can include providingmoisture treated silicon particles. For example, the moisture treatedsilicon particles can comprise silicon particles treated with water,silicon particles treated with alcohol, liquid-boiled silicon particles,liquid-decanted silicon particles, steamed silicon particles, siliconparticles heat treated with moisture, and/or silicon particles treatedwith an oxidizing reagent. The oxidizing reagent can include potassiumhydroxide and/or hydrogen peroxide. In some instances of the method,providing the mixture can include providing moisture to the precursor.The precursor can comprise a polymer and a solvent. Providing moistureto the precursor can include providing moisture to the polymer and/orproviding moisture to the solvent. The solvent can includeN-methyl-pyrrolidone (NMP) and/or water. The precursor can include awater-soluble polymer.

In some embodiments of the method, the silicon carbide and/or one of theone or more types of carbon phases can form substantially continuouslayers on the silicon particles. Forming silicon carbide can includereacting one of the one or more types of carbon phases with the siliconparticles. Reacting one of the one or more types of carbon phases withthe silicon particles can include reacting one or more types of carbonphases with native silicon oxide layers of the silicon particles.Pyrolyzing the precursor can include heating the mixture to atemperature of about 500° C. to about 1300° C. For example, pyrolyzingthe precursor can include heating the mixture to a temperature of about800° C. to about 1200° C. As another example, pyrolyzing the precursorcan include heating the mixture to a temperature of about 1175° C. Insome instances, pyrolyzing can comprise providing moisture to themixture.

In some embodiments of the method, the method can comprise casting themixture on a substrate, drying the mixture to form a film, removing thefilm from the substrate, and placing the film in a hot press. Castingthe mixture can include providing moisture to the mixture. Drying themixture can include heat treating the mixture with moisture. Placing thefilm in a hot press can include providing moisture to the film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an embodiment of a method of forming a compositematerial that includes forming a mixture that includes a precursor,casting the mixture, drying the mixture, curing the mixture, andpyrolyzing the precursor;

FIG. 1B is a schematic illustration of the formation of silicon carbideon a silicon particle;

FIGS. 2A and 2B are SEM micrographs of one embodiment of micron-sizedsilicon particles milled-down from larger silicon particles;

FIGS. 2C and 2D are SEM micrographs of one embodiment of micron-sizedsilicon particles with nanometer-sized features on the surface;

FIG. 2E illustrates an example embodiment of a method of forming acomposite material;

FIG. 3A schematically illustrates an example moisture treated siliconparticle;

FIG. 3B illustrates an example embodiment of a method of forming acomposite material;

FIG. 4 is a plot of the discharge capacity at an average rate of C/2.6;

FIG. 5 is a plot of the discharge capacity at an average rate of C/3;

FIG. 6 is a plot of the discharge capacity at an average rate of C/3.3;

FIG. 7 is a plot of the discharge capacity at an average rate of C/5;

FIG. 8 is a plot of the discharge capacity at an average rate of C/9;

FIG. 9 is a plot of the discharge capacity;

FIG. 10 is a plot of the discharge capacity at an average rate of C/9;

FIGS. 11A and 11B are plots of the reversible and irreversible capacityas a function of the various weight percentage of PI derived carbon from2611 c and graphite particles for a fixed percentage of 20 wt. % Si;

FIG. 12 is a plot of the first cycle discharge capacity as a function ofweight percentage of carbon;

FIG. 13 is a plot of the reversible (discharge) and irreversiblecapacity as a function of pyrolysis temperature;

FIG. 14 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer;

FIG. 15 is a scanning electron microscope (SEM) micrograph of acomposite anode film before being cycled (the out-of-focus portion is abottom portion of the anode and the portion that is in focus is acleaved edge of the composite film);

FIG. 16 is another SEM micrograph of a composite anode film before beingcycled;

FIG. 17 is a SEM micrograph of a composite anode film after being cycled10 cycles;

FIG. 18 is another SEM micrograph of a composite anode film after beingcycled 10 cycles;

FIG. 19 is a SEM micrograph of a composite anode film after being cycled300 cycles;

FIG. 20 includes SEM micrographs of cross-sections of composite anodefilms;

FIG. 21 is an x-ray powder diffraction (XRD) graph of the sample siliconparticles;

FIG. 22 is a SEM micrograph of one embodiment of silicon particles;

FIG. 23 is another SEM micrographs of one embodiment of siliconparticles;

FIG. 24 is a SEM micrograph of one embodiment of silicon particles;

FIG. 25 is a SEM micrograph of one embodiment of silicon particles;

FIG. 26 is a chemical analysis of the sample silicon particles;

FIGS. 27A and 27B are example particle size histograms of twomicron-sized silicon particles with nanometer-sized features;

FIG. 28 is a plot of discharge capacity during cell cycling comparingtwo types of example silicon particles;

FIG. 29 shows X-ray Photoelectron Spectroscopy (XPS) spectra of thealuminum 2p peak in example moisture treated silicon particles comparedwith untreated silicon particles; and

FIG. 30 shows a graph of capacity retention versus cycle number for abattery comprising moisture treated silicon particles compared with abattery comprising untreated silicon particles.

DETAILED DESCRIPTION

This application describes certain embodiments of silicon material thatmay be used as electrochemically active material in electrodes (e.g.,anodes and cathodes) in electrochemical cells. Silicon can be apotentially high energy per unit volume host material, such as forlithium ion batteries. For example, silicon has a high theoreticalcapacity and can increase the energy density of lithium ion batteriescompared with lithium ion batteries using other active materials such asgraphite. However, silicon can swell in excess of 300% upon lithiuminsertion. Accordingly, batteries with silicon anodes may exhibit morerapid capacity loss upon cycling compared with batteries with graphiteanodes. The repeat expansion and contraction of silicon particles duringcharge and discharge can lead to mechanical failure of the anode duringcycling. Mechanical failure can expose new surfaces of silicon which canreact with the electrolyte, irreversibly incorporating lithium ions inthe solid electrolyte interface/interphase (SEI), and leading tocapacity loss. Certain embodiments described herein can include siliconmaterial with a modified surface, leading to improved cyclingperformance. For example, some embodiments can provide an SEI withincreased stability (e.g., a substantially stable SEI and/or a stableSEI in some instances) to improve the capacity retention and reduce(e.g., and/or prevent in some instances) fast fading.

Typical carbon anode electrodes include a current collector such as acopper sheet. Carbon is deposited onto the collector along with aninactive binder material. Carbon is often used because it has excellentelectrochemical properties and is also electrically conductive. If thecurrent collector layer (e.g., copper layer) was removed, the carbonwould likely be unable to mechanically support itself. Therefore,conventional electrodes require a support structure such as thecollector to be able to function as an electrode. The electrode (e.g.,anode or cathode) compositions described in this application can produceelectrodes that are self-supported. The need for a metal foil currentcollector is eliminated or minimized because conductive carbonizedpolymer is used for current collection in the anode structure as well asfor mechanical support. In typical applications for the mobile industry,a metal current collector is typically added to ensure sufficient rateperformance. The carbonized polymer can form a substantially continuousconductive carbon phase in the entire electrode as opposed toparticulate carbon suspended in a non-conductive binder in one class ofconventional lithium-ion battery electrodes. Advantages of a carboncomposite blend that utilizes a carbonized polymer can include, forexample, 1) higher capacity, 2) enhanced overcharge/dischargeprotection, 3) lower irreversible capacity due to the elimination (orminimization) of metal foil current collectors, and 4) potential costsavings due to simpler manufacturing.

Anode electrodes currently used in the rechargeable lithium-ion cellstypically have a specific capacity of approximately 200 milliamp hoursper gram (including the metal foil current collector, conductiveadditives, and binder material). Graphite, the active material used inmost lithium ion battery anodes, has a theoretical energy density of 372milliamp hours per gram (mAh/g). In comparison, silicon has a hightheoretical capacity of 4200 mAh/g. In order to increase volumetric andgravimetric energy density of lithium-ion batteries, silicon may be usedas the active material for the cathode or anode. Several types ofsilicon materials, e.g., silicon nanopowders, silicon nanofibers, poroussilicon, and ball-milled silicon, have also been reported as viablecandidates as active materials for the negative or positive electrode.Small particle sizes (for example, sizes in the nanometer range)generally can increase cycle life performance. They also can displayvery high irreversible capacity. However, small particle sizes also canresult in very low volumetric energy density (for example, for theoverall cell stack) due to the difficulty of packing the activematerial. Larger particle sizes, (for example, sizes in the micrometeror micron range) generally can result in higher density anode material.However, the expansion of the silicon active material can result in poorcycle life due to particle cracking. For example, silicon can swell inexcess of 300% upon lithium insertion. Because of this expansion, anodesincluding silicon should be allowed to expand while maintainingelectrical contact between the silicon particles.

As described herein and in U.S. patent application Ser. No. 13/008,800(U.S. Pat. No. 9,178,208) and Ser. No. 13/601,976 (U.S. PatentApplication Publication No. 2014/0170498), entitled “Composite Materialsfor Electrochemical Storage” and “Silicon Particles for BatteryElectrodes,” respectively, the entireties of which are herebyincorporated by reference, certain embodiments utilize a method ofcreating monolithic, self-supported anodes using a carbonized polymer.Because the polymer is converted into an electrically conductive andelectrochemically active matrix, the resulting electrode is conductiveenough that a metal foil or mesh current collector can be omitted orminimized. The converted polymer also acts as an expansion buffer forsilicon particles during cycling so that a high cycle life can beachieved. In certain embodiments, the resulting electrode is anelectrode that is comprised substantially of active material. In furtherembodiments, the resulting electrode is substantially active material.The electrodes can have a high energy density of between about 500 mAh/gto about 3500 mAh/g that can be due to, for example, 1) the use ofsilicon, 2) elimination or substantial reduction of metal currentcollectors, and 3) being comprised entirely or substantially entirely ofactive material.

As described in U.S. patent application Ser. No. 14/821,586 (U.S. PatentApplication Publication No. 2017/0040598), entitled “SurfaceModification of Silicon Particles for Electrochemical Storage,” theentirety of which is hereby incorporated by reference, in certainembodiments, carbonized polymer may react with a native silicon oxidesurface layer on the silicon particles. In some embodiments, the surfaceof the particles is modified to form a surface coating thereon, whichmay further act as an expansion buffer for silicon particles duringcycling. The surface coating may include silicon carbide.

The composite materials described herein can be used as an anode in mostconventional lithium ion batteries; they may also be used as the cathodein some electrochemical couples with additional additives. The compositematerials can also be used in either secondary batteries (e.g.,rechargeable) or primary batteries (e.g., non-rechargeable). In certainembodiments, the composite materials are self-supported structures. Infurther embodiments, the composite materials are self-supportedmonolithic structures. For example, a collector may be included in theelectrode comprised of the composite material. In certain embodiments,the composite material can be used to form carbon structures discussedin U.S. patent application Ser. No. 12/838,368 (U.S. Patent ApplicationPublication No. 2011/0020701), entitled “Carbon Electrode Structures forBatteries,” the entirety of which is hereby incorporated by reference.Furthermore, the composite materials described herein can be, forexample, silicon composite materials, carbon composite materials, and/orsilicon-carbon composite materials. As described in U.S. patentapplication Ser. No. 13/799,405 (U.S. Pat. No. 9,553,303), entitled“Silicon Particles for Battery Electrodes,” the entirety of which ishereby incorporated by reference, certain embodiments can furtherinclude composite materials including micron-sized silicon particles.For example, in some embodiments, the micron-sized silicon particleshave nanometer-sized features on the surface. Silicon particles withsuch a geometry may have the benefits of both micron-sized siliconparticles (e.g., high energy density) and nanometer-sized siliconparticles (e.g., good cycling behavior). As used herein, the term“silicon particles” in general can include micron-sized siliconparticles with or without nanometer-sized features.

FIG. 1A illustrates one embodiment of a method of forming a compositematerial 100. For example, the method of forming a composite materialcan include forming a mixture including a precursor, block 101. Themethod can further include pyrolyzing the precursor to convert theprecursor to a carbon phase. The precursor mixture may include carbonadditives such as graphite active material, chopped or milled carbonfiber, carbon nanofibers, carbon nanotubes, and/or other carbons. Afterthe precursor is pyrolyzed, the resulting carbon material can be aself-supporting monolithic structure. In certain embodiments, one ormore materials are added to the mixture to form a composite material.For example, silicon particles can be added to the mixture. Thecarbonized precursor results in an electrochemically active structurethat holds the composite material together. For example, the carbonizedprecursor can be a substantially continuous phase. The siliconparticles, including micron-sized silicon particles with or withoutnanometer-sized features, may be distributed throughout the compositematerial. Advantageously, the carbonized precursor can be a structuralmaterial as well as an electro-chemically active and electricallyconductive material. In certain embodiments, material particles added tothe mixture are homogenously or substantially homogeneously distributedthroughout the composite material to form a homogeneous or substantiallyhomogeneous composite.

The mixture can include a variety of different components. The mixturecan include one or more precursors. In certain embodiments, theprecursor is a hydrocarbon compound. For example, the precursor caninclude polyamideimide, polyamic acid, polyimide, etc. Other precursorscan include phenolic resins, epoxy resins, and/or other polymers. Themixture can further include a solvent. For example, the solvent can beN-methyl-pyrrolidone (NMP). Other possible solvents include acetone,diethyl ether, gamma butyrolactone, isopropanol, dimethyl carbonate,ethyl carbonate, dimethoxyethane, ethanol, methanol, etc. Examples ofprecursor and solvent solutions include PI-2611 (HD Microsystems),PI-5878G (HD Microsystems) and VTEC PI-1388 (RBI, Inc.). PI-2611 iscomprised of >60% n-methyl-2-pyrrolidone and 10-30%s-biphenyldianhydride/p-phenylenediamine. PI-5878G is comprised of >60%n-methylpyrrolidone, 10-30% polyamic acid of pyromelliticdianhydride/oxydianiline, 10-30% aromatic hydrocarbon (petroleumdistillate) including 5-10% 1,2,4-trimethylbenzene. In certainembodiments, the amount of precursor in the solvent is about 10 wt. % toabout 30 wt. %. Additional materials can also be included in themixture. For example, as previously discussed, silicon particles orcarbon particles including graphite active material, chopped or milledcarbon fiber, carbon nanofibers, carbon nanotubes, and other conductivecarbons can be added to the mixture. In addition, the mixture can bemixed to homogenize the mixture.

In certain embodiments, the mixture is cast on a substrate, block 102 inFIG. 1A. In some embodiments, casting includes using a gap extrusion,tape casting, or a blade casting technique. The blade casting techniquecan include applying a coating to the substrate by using a flat surface(e.g., blade) which is controlled to be a certain distance above thesubstrate. A liquid or slurry can be applied to the substrate, and theblade can be passed over the liquid to spread the liquid over thesubstrate. The thickness of the coating can be controlled by the gapbetween the blade and the substrate since the liquid passes through thegap. As the liquid passes through the gap, excess liquid can also bescraped off. For example, the mixture can be cast on a substratecomprising a polymer sheet, a polymer roll, and/or foils or rolls madeof glass or metal. The mixture can then be dried to remove the solvent,block 103. For example, a polyamic acid and NMP solution can be dried atabout 110° C. for about 2 hours to remove the NMP solution. The driedmixture can then be removed from the substrate. For example, an aluminumsubstrate can be etched away with HCl. Alternatively, the dried mixturecan be removed from the substrate by peeling or otherwise mechanicallyremoving the dried mixture from the substrate. In some embodiments, thesubstrate comprises polyethylene terephthalate (PET), including forexample Mylar®. In certain embodiments, the dried mixture is a film orsheet. In some embodiments, the dried mixture is optionally cured, block104. In some embodiments, the dried mixture may be further dried. Forexample, the dried mixture can placed in a hot press (e.g., betweengraphite plates in an oven). A hot press can be used to further dryand/or cure and to keep the dried mixture flat. For example, the driedmixture from a polyamic acid and NMP solution can be hot pressed atabout 200° C. for about 8 to 16 hours. Alternatively, the entire processincluding casting and drying can be done as a roll-to-roll process usingstandard film-handling equipment. The dried mixture can be rinsed toremove any solvents or etchants that may remain. For example, de-ionized(DI) water can be used to rinse the dried mixture. In certainembodiments, tape casting techniques can be used for the casting. Insome embodiments, the mixture can be coated on a substrate by a slot diecoating process (e.g., metering a constant or substantially constantweight and/or volume through a set or substantially set gap). In someother embodiments, there is no substrate for casting and the anode filmdoes not need to be removed from any substrate. The dried mixture may becut or mechanically sectioned into smaller pieces.

The mixture further goes through pyrolysis to convert the polymerprecursor to carbon, block 105. In certain embodiments, the mixture ispyrolysed in a reducing atmosphere. For example, an inert atmosphere, avacuum and/or flowing argon, nitrogen, or helium gas can be used. Insome embodiments, the mixture is heated to about 900° C. to about 1350°C. For example, polyimide formed from polyamic acid can be carbonized atabout 1175° C. for about one hour. In certain embodiments, the heat uprate and/or cool down rate of the mixture is about 10° C./min. A holdermay be used to keep the mixture in a particular geometry. The holder canbe graphite, metal, etc. In certain embodiments, the mixture is heldflat. After the mixture is pyrolysed, tabs can be attached to thepyrolysed material to form electrical contacts. For example, nickel,copper or alloys thereof can be used for the tabs.

In certain embodiments, one or more of the methods described herein canbe carried out in a continuous process. In certain embodiments, casting,drying, possibly curing and pyrolysis can be performed in a continuousprocess. For example, the mixture can be coated onto a glass or metalcylinder. The mixture can be dried while rotating on the cylinder tocreate a film. The film can be transferred as a roll or peeled and fedinto another machine for further processing. Extrusion and other filmmanufacturing techniques known in industry could also be utilized priorto the pyrolysis step.

Pyrolysis of the precursor results in a carbon material (e.g., at leastone carbon phase). In certain embodiments, the carbon material is a hardcarbon. In some embodiments, the precursor is any material that can bepyrolysed to form a hard carbon. When the mixture includes one or moreadditional materials or phases in addition to the carbonized precursor,a composite material can be created. In particular, the mixture caninclude silicon particles, creating a silicon-carbon (e.g., at least onefirst phase comprising silicon and at least one second phase comprisingcarbon) or silicon-carbon-carbon (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast one third phase comprising carbon) composite material.

Silicon particles can increase the specific lithium insertion capacityof the composite material. When silicon absorbs lithium ions, itexperiences a large volume increase on the order of 300+ volume percentwhich can cause electrode structural integrity issues. In addition tovolumetric expansion related problems, silicon is not inherentlyelectrically conductive, but becomes conductive when it is alloyed withlithium (e.g., lithiation). When silicon de-lithiates, the surface ofthe silicon loses electrical conductivity. Furthermore, when siliconde-lithiates, the volume decreases which results in the possibility ofthe silicon particle losing contact with the matrix. The dramatic changein volume also results in mechanical failure of the silicon particlestructure, in turn, causing it to pulverize. Pulverization and loss ofelectrical contact have made it a challenge to use silicon as an activematerial in lithium-ion batteries. A reduction in the initial size ofthe silicon particles can prevent further pulverization of the siliconpowder as well as minimizing the loss of surface electricalconductivity. Furthermore, adding material to the composite that canelastically deform with the change in volume of the silicon particlescan reduce the chance that electrical contact to the surface of thesilicon is lost. For example, the composite material can include carbonssuch as graphite which contributes to the ability of the composite toabsorb expansion and which is also capable of intercalating lithium ionsadding to the storage capacity of the electrode (e.g., chemicallyactive). Therefore, the composite material may include one or more typesof carbon phases.

In some embodiments, the particle size (e.g., diameter or a largestdimension of the silicon particles) can be less than about 50 μm, lessthan about 40 μm, less than about 30 μm, less than about 20 μm, lessthan about 10 μm, less than about 1 μm, between about 10 nm and about 50μm, between about 10 nm and about 40 μm, between about 10 nm and about30 μm, between about 10 nm and about 20 μm, between about 0.1 μm andabout 20 μm, between about 0.5 μm and about 20 μm, between about 1 μmand about 20 μm, between about 1 μm and about 15 μm, between about 1 μmand about 10 μm, between about 10 nm and about 10 μm, between about 10nm and about 1 μm, less than about 500 nm, less than about 100 nm, about100 nm, etc. All, substantially all, or at least some of the siliconparticles may comprise the particle size (e.g., diameter or largestdimension) described above. For example, an average particle size (orthe average diameter or the average largest dimension) or a medianparticle size (or the median diameter or the median largest dimension)of the silicon particles can be less than about 50 μm, less than about40 μm, less than about 30 μm, less than about 20 μm, less than about 10μm, less than about 1 μm, between about 10 nm and about 50 μm, betweenabout 10 nm and about 40 μm, between about 10 nm and about 30 μm,between about 10 nm and about 20 μm, between about 0.1 μm and about 20μm, between about 0.5 μm and about 20 μm, between about 1 μm and about20 μm, between about 1 μm and about 15 μm, between about 1 μm and about10 μm, between about 10 nm and about 10 μm, between about 10 nm andabout 1 μm, less than about 500 nm, less than about 100 nm, about 100nm, etc. In some embodiments, the silicon particles may have adistribution of particle sizes. For example, at least about 95%, atleast about 90%, at least about 85%, at least about 80%, at least about70%, or at least about 60% of the particles may have the particle sizedescribed herein.

The amount of silicon provided in the mixture or in the compositematerial can be greater than zero percent by weight of the mixtureand/or composite material. In certain embodiments, the amount of siliconcan be within a range of from about 0% to about 99% by weight of thecomposite material, including greater than about 0% to about 99% byweight, greater than about 0% to about 95% by weight, greater than about0% to about 90%, greater than about 0% to about 35% by weight, greaterthan about 0% to about 25% by weight, from about 10% to about 35% byweight, at least about 30% by weight, from about 30% to about 99% byweight, from about 30% to about 95% by weight, from about 30% to about90% by weight, from about 30% to about 80% by weight, at least about 50%by weight, from about 50% to about 99% by weight, from about 50% toabout 95% by weight, from about 50% to about 90% by weight, from about50% to about 80% by weight, from about 50% to about 70% by weight, atleast about 60% by weight, from about 60% to about 99% by weight, fromabout 60% to about 95% by weight, from about 60% to about 90% by weight,from about 60% to about 80% by weight, at least about 70% by weight,from about 70% to about 99% by weight, from about 70% to about 95% byweight, from about 70% to about 90% by weight, etc.

Furthermore, the silicon particles may or may not be pure silicon. Forexample, the silicon particles may be substantially silicon or may be asilicon alloy. In one embodiment, the silicon alloy includes silicon asthe primary constituent along with one or more other elements. Asdescribed herein, in some embodiments, the silicon particles may containaluminum.

As described herein, micron-sized silicon particles can provide goodvolumetric and gravimetric energy density combined with good cycle life.In certain embodiments, to obtain the benefits of both micron-sizedsilicon particles (e.g., high energy density) and nanometer-sizedsilicon particles (e.g., good cycle behavior), silicon particles canhave an average particle size or a median particle size in the micronrange and a surface including nanometer-sized features. In someembodiments, the silicon particles can have an average particle size(e.g., average diameter or average largest dimension) or a medianparticle size (e.g., median diameter or median largest diameter) betweenabout 0.1 μm and about 30 μm or between about 0.1 μm and all values upto about 30 μm. For example, the silicon particles can have an averageparticle size or a median particle size between about 0.1 μm and about20 μm, between about 0.5 μm and about 25 μm, between about 0.5 μm andabout 20 μm, between about 0.5 μm and about 15 μm, between about 0.5 μmand about 10 μm, between about 0.5 μm and about 5 μm, between about 0.5μm and about 2 μm, between about 1 μm and about 20 μm, between about 1μm and about 15 μm, between about 1 μm and about 10 μm, between about 5μm and about 20 μm, etc. Thus, the average particle size or the medianparticle size can be any value between about 0.1 μm and about 30 μm,e.g., 0.1 μm, 0.5 μm, 1 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 15 μm,20 μm, 25 μm, and 30 μm.

The nanometer-sized features can include an average feature size (e.g.,an average diameter or an average largest dimension) between about 1 nmand about 1 μm, between about 1 nm and about 750 nm, between about 1 nmand about 500 nm, between about 1 nm and about 250 nm, between about 1nm and about 100 nm, between about 10 nm and about 500 nm, between about10 nm and about 250 nm, between about 10 nm and about 100 nm, betweenabout 10 nm and about 75 nm, or between about 10 nm and about 50 nm. Thefeatures can include silicon.

The amount of carbon obtained from the precursor can be about 50 weightpercent from polyamic acid. In certain embodiments, the amount of carbonobtained from the precursor in the composite material can be greaterthan 0% to about 95% by weight such as about 1% to about 95% by weight,about 1% to about 90% by weight, 1% to about 80% by weight, about 1% toabout 70% by weight, about 1% to about 60% by weight, about 1% to about50% by weight, about 1% to about 40% by weight, about 1% to about 30% byweight, about 5% to about 95% by weight, about 5% to about 90% byweight, about 5% to about 80% by weight, about 5% to about 70% byweight, about 5% to about 60% by weight, about 5% to about 50% byweight, about 5% to about 40% by weight, about 5% to about 30% byweight, about 10% to about 95% by weight, about 10% to about 90% byweight, about 10% to about 80% by weight, about 10% to about 70% byweight, about 10% to about 60% by weight, about 10% to about 50% byweight, about 10% to about 40% by weight, about 10% to about 30% byweight, about 10% to about 25% by weight, etc. For example, the amountof carbon obtained from the precursor can be about 1%, about 5%, about10% by weight, about 15% by weight, about 20% by weight, about 25% byweight, etc. from the precursor.

The carbon from the precursor can be hard carbon. Hard carbon can be acarbon that does not convert into graphite even with heating in excessof 2800 degrees Celsius. Precursors that melt or flow during pyrolysisconvert into soft carbons and/or graphite with sufficient temperatureand/or pressure. Hard carbon may be selected since soft carbonprecursors may flow and soft carbons and graphite are mechanicallyweaker than hard carbons. Other possible hard carbon precursors caninclude phenolic resins, epoxy resins, and other polymers that have avery high melting point or are crosslinked. In some embodiments, theamount of hard carbon in the composite material can have a value withina range of greater than 0% to about 95% by weight such as about 1% toabout 95% by weight, about 1% to about 90% by weight, about 1% to about80% by weight, about 1% to about 70% by weight, about 1% to about 60% byweight, about 1% to about 50% by weight, about 1% to about 40% byweight, about 1% to about 30% by weight, about 5% to about 95% byweight, about 5% to about 90% by weight, about 5% to about 80% byweight, about 5% to about 70% by weight, about 5% to about 60% byweight, about 5% to about 50% by weight, about 5% to about 40% byweight, about 5% to about 30% by weight, about 10% to about 95% byweight, about 10% to about 90% by weight, about 10% to about 80% byweight, about 10% to about 70% by weight, about 10% to about 60% byweight, about 10% to about 50% by weight, about 10% to about 40% byweight, about 10% to about 30% by weight, about 10% to about 25% byweight, etc. In some embodiments, the amount of hard carbon in thecomposite material can be about 1% by weight, about 5% by weight, about10% by weight, about 20% by weight, about 30% by weight, about 40% byweight, about 50% by weight, or more than about 50% by weight. Incertain embodiments, the hard carbon phase is substantially amorphous.In other embodiments, the hard carbon phase is substantiallycrystalline. In further embodiments, the hard carbon phase includesamorphous and crystalline carbon. The hard carbon phase can be a matrixphase in the composite material. The hard carbon can also be embedded inthe pores of the additives including silicon. The hard carbon may reactwith some of the additives to create some materials at interfaces. Forexample, there may be a silicon carbide layer between silicon particlesand the hard carbon.

In certain embodiments heating the mixture to a desired pyrolysistemperature may further result in the surface modification of siliconparticles present in the mixture. In some embodiments pyrolysis of themixture may result in the formation of a surface coating on at least 50%of the silicon particles present in the mixture. In some embodimentspyrolysis of the mixture may result in the formation of a surfacecoating on at least 60%, 70%, 80%, 90% or 99% of the silicon particlespresent in the mixture. In some embodiments, the surface coatings form asubstantially continuous layer on the silicon particles.

In some embodiments, the carbonized precursor or resin may contact thesurface of the silicon particles. In certain embodiments, the carbonizedprecursor in contact with the silicon particle surface may be one ormore types of carbon phases resulting from pyrolysis of the precursor.The one or more types of carbon phases of the carbonized precursor incontact with the silicon particle surface may react with the siliconparticles during pyrolysis to thereby form silicon carbide on thesilicon particle surface. Therefore, in some embodiments, the surfacecoatings may comprise carbon, silicon carbide, and/or a mixture ofcarbon and silicon carbide.

In some embodiments, as described further below, the silicon particlespresent in the mixture may comprise a native silicon oxide (SiO, SiO₂,SiOx) surface layer. In certain embodiments, the carbonized precursor incontact with the silicon particle surface may react with the naturallyoccurring native silicon oxide surface layer to form silicon carbide. Insome embodiments the carbonized precursor in contact with the siliconparticle surface may react with substantially all of the native siliconoxide layer to form silicon carbide. Therefore, the surface coatings onthe silicon particles may comprise, in some embodiments, carbon andsilicon carbide, wherein the surface coating is substantially free ofsilicon oxide. In some embodiments a first portion of the surfacecoatings may comprise silicon carbide while a second portion maycomprise a mixture of silicon carbide and carbon. In some otherembodiments, the carbonized precursor in contact with the siliconparticle surface may not fully convert the native silicon oxide layer tosilicon carbide, and the resultant surface coating or coatings maycomprise carbon, silicon carbide, and one or more silicon oxides, suchas SiO, SiO₂, and SiO_(x). In some embodiments, the carbonized precursorin contact with the silicon particle surface may be completely reacted,resulting in surface coatings that comprise silicon carbide. In someembodiments substantially all of the surface coatings may comprisesilicon carbide. In some embodiments, such surface coatings may besubstantially free of silicon oxide and/or carbon.

In certain embodiments, the pyrolyzed mixture can include siliconparticles having carbon and/or silicon carbide surface coatings creatinga silicon-carbon-silicon carbide (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast a third phase comprising silicon carbide) orsilicon-carbon-carbon-silicon carbide (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, atleast one third phase comprising carbon, and at least a fourth phasecomprising silicon carbide) composite material.

Additionally, surface coatings on the silicon particles described hereincan help to constrain the outward expansion of the silicon particleduring lithiation. By constraining outward particle expansion duringlithiation, the surface coatings can help prevent mechanical failure ofthe silicon particles and ensure good electrical contact. The surfacecoatings can further enhance the electronic charge transfer within theelectrode. Controlled and optimized surface modification of siliconparticles in the anode may also significantly improve capacity retentionduring cycling of an associated battery cell.

Moreover, the surface coatings substantially affect the reactions thatoccur between the anode materials and the electrolyte within a battery.The surface coatings can help reduce unwanted reactions. During hightemperature pyrolysis, the formed surface coatings and the removal ofunwanted native oxide (SiO₂) via conversion into more stable andunreactive SiC can provide higher reversible capacity with minimizedirreversible capacity loss. Irreversible capacity loss can be due toformation and build-up of a solid electrolyte interface (SEI) layer thatconsumes lithium. This becomes a more prominent issue for siliconparticles because nano- and micro-scale silicon particles have largesurface areas and larger silicon particles tend to pulverize duringlithiation and delithiation which can introduce additional particlesurface area. Additionally, irreversible capacity loss can be due to thereaction of lithium with undesirable native silicon oxides (Equation 1)which are unavoidable during processing and storage of silicon anodematerials.

SiO_(x) +yLi+ye→Si+Li_(y)O_(x)  (Equation 1)

Therefore, the surface modification of the silicon particles by carbonand/or silicon carbide may aid in the formation of a relatively stablesolid electrolyte interface layer and may reduce or eliminate theundesirable reaction of lithium with native silicon oxides on the Siparticle surface (Equation 1).

FIG. 1B is a schematic illustration of the formation of silicon carbideon a silicon particle as described above. Initially, a silicon particlecomprising a native silicon oxide surface layer is provided in a mixturecomprising a precursor as described above. In some embodiments, themixture is pyrolyzed in a reducing atmosphere. For example, a reducingatmosphere, a vacuum and/or flowing gas including H₂, CO, or hydrocarbongas can be used. In some embodiments, the mixture is heated to about500° C. to about 1350° C. In some embodiments, the mixture is heated toabout 800° C. to about 1200° C. In some embodiments, the mixture isheated to about 1175° C.

The pyrolyzed precursor in contact with the surface of the siliconparticle reacts with the native silicon oxide layer of the siliconparticle to form silicon carbide. The carbonized precursor in contactwith the silicon particle surface is depicted here as continuous andconformal, but may not be continuous or conformal in some otherembodiments. Further, in some embodiments, the silicon carbide layerformed from the reaction between the native silicon oxide layer and thecarbonized precursor in contact with the silicon particle surface maytake the form of a coating or dispersion within the composite anodefilm. As shown in FIG. 1B, in some embodiments the silicon carbide maynot be continuous or conformal on the silicon particle, however in someother embodiments the silicon carbide may be a continuous and/orconformal coating.

In certain embodiments, graphite particles are added to the mixture.Advantageously, graphite can be an electrochemically active material inthe battery as well as an elastic deformable material that can respondto volume change of the silicon particles. Graphite is the preferredactive anode material for certain classes of lithium-ion batteriescurrently on the market because it has a low irreversible capacity.Additionally, graphite is softer than hard carbon and can better absorbthe volume expansion of silicon additives. In certain embodiments, theparticle size (e.g., a diameter or a largest dimension) of the graphiteparticles can be between about 0.5 microns and about 20 microns. All,substantially all, or at least some of the graphite particles maycomprise the particle size (e.g., diameter or largest dimension)described herein. In some embodiments, an average or median particlesize (e.g., diameter or largest dimension) of the graphite particles canbe between about 0.5 microns and about 20 microns. In some embodiments,the graphite particles may have a distribution of particle sizes. Forexample, at least about 95%, at least about 90%, at least about 85%, atleast about 80%, at least about 70%, or at least about 60% of theparticles may have the particle size described herein. In certainembodiments, the composite material can include graphite particles in anamount greater than 0% and less than about 80% by weight, including fromabout 40% to about 75% by weight, from about 5% to about 30% by weight,from about 5% to about 25% by weight, from about 5% to about 20% byweight, from about 5% to about 15% by weight, etc.

In certain embodiments, conductive particles which may also beelectrochemically active are added to the mixture. Such particles canenable both a more electronically conductive composite as well as a moremechanically deformable composite capable of absorbing the largevolumetric change incurred during lithiation and de-lithiation. Incertain embodiments, a particle size (e.g., diameter or a largestdimension) of the conductive particles can be between about 10nanometers and about 7 micrometers. All, substantially all, or at leastsome of the conductive particles may comprise the particle size (e.g.,diameter or largest dimension) described herein. In some embodiments, anaverage or median particle size (e.g., diameter or largest dimension) ofthe conductive particles can be between about 10 nm and about 7micrometers. In some embodiments, the conductive particles may have adistribution of particle sizes. For example, at least about 95%, atleast about 90%, at least about 85%, at least about 80%, at least about70%, or at least about 60% of the particles may have the particle sizedescribed herein.

In certain embodiments, the mixture can include conductive particles inan amount greater than zero and up to about 80% by weight. In furtherembodiments, the composite material can include about 45% to about 80%by weight. The conductive particles can be conductive carbon includingcarbon blacks, carbon fibers, carbon nanofibers, carbon nanotubes, etc.Many carbons that are considered as conductive additives that are notelectrochemically active become active once pyrolyzed in a polymermatrix. Alternatively, the conductive particles can be metals or alloysincluding copper, nickel, or stainless steel.

In certain embodiments, an electrode can include a composite materialdescribed herein. For example, a composite material can form aself-supported monolithic electrode. The pyrolysed carbon phase (e.g.,hard carbon phase) of the composite material can hold together andstructurally support the particles that were added to the mixture. Incertain embodiments, the self-supported monolithic electrode does notinclude a separate collector layer and/or other supportive structures.In some embodiments, the composite material and/or electrode does notinclude a polymer beyond trace amounts that remain after pyrolysis ofthe precursor. In further embodiments, the composite material and/orelectrode does not include a non-electrically conductive binder. Thecomposite material may also include porosity, such as about 1% to about70% or about 5% to about 50% by volume porosity. For example, theporosity can be about 5% to about 40% by volume porosity.

The composite material may also be formed into a powder. For example,the composite material can be ground into a powder. The compositematerial powder can be used as an active material for an electrode. Forexample, the composite material powder can be deposited on a collectorin a manner similar to making a conventional electrode structure, asknown in the industry.

In certain embodiments, an electrode in a battery or electrochemicalcell can include a composite material, including composite material withthe silicon particles described herein. For example, the compositematerial can be used for the anode and/or cathode. In certainembodiments, the battery is a lithium ion battery. In furtherembodiments, the battery is a secondary battery, or in otherembodiments, the battery is a primary battery.

Furthermore, the full capacity of the composite material may not beutilized during use of the battery to improve life of the battery (e.g.,number charge and discharge cycles before the battery fails or theperformance of the battery decreases below a usability level). Forexample, a composite material with about 70% by weight siliconparticles, about 20% by weight carbon from a precursor, and about 10% byweight graphite may have a maximum gravimetric capacity of about 3000mAh/g, while the composite material may only be used up to angravimetric capacity of about 550 to about 1500 mAh/g. Although, themaximum gravimetric capacity of the composite material may not beutilized, using the composite material at a lower capacity can stillachieve a higher capacity than certain lithium ion batteries. In certainembodiments, the composite material is used or only used at angravimetric capacity below about 70% of the composite material's maximumgravimetric capacity. For example, the composite material is not used atan gravimetric capacity above about 70% of the composite material'smaximum gravimetric capacity. In further embodiments, the compositematerial is used or only used at an gravimetric capacity below about 50%of the composite material's maximum gravimetric capacity or below about30% of the composite material's maximum gravimetric capacity.

Silicon Particles

Described herein are silicon particles for use in battery electrodes(e.g., anodes and cathodes). Anode electrodes currently used in therechargeable lithium-ion cells typically have a specific capacity ofapproximately 200 milliamp hours per gram (including the metal foilcurrent collector, conductive additives, and binder material). Graphite,the active material used in most lithium ion battery anodes, has atheoretical energy density of 372 milliamp hours per gram (mAh/g). Incomparison, silicon has a high theoretical capacity of 4200 mAh/g.Silicon, however, swells in excess of 300% upon lithium insertion.Because of this expansion, anodes including silicon should be able toexpand while allowing for the silicon to maintain electrical contactwith the silicon.

Some embodiments provide silicon particles that can be used as anelectro-chemically active material in an electrode. The electrode mayinclude binders and/or other electro-chemically active materials inaddition to the silicon particles. For example, the silicon particlesdescribed herein can be used as the silicon particles in the compositematerials described herein. In another example, an electrode can have anelectro-chemically active material layer on a current collector, and theelectro-chemically active material layer includes the silicon particles.The electro-chemically active material may also include one or moretypes of carbon.

Advantageously, the silicon particles described herein can improveperformance of electro-chemically active materials such as improvingcapacity and/or cycling performance. Furthermore, electro-chemicallyactive materials having such silicon particles may not significantlydegrade as a result of lithiation of the silicon particles.

In certain embodiments, the silicon particles can have an averageparticle size, for example an average diameter or an average largestdimension, between about 10 nm and about 40 μm as described herein.Further embodiments can include average particle sizes of between about1 μm and about 15 μm, between about 10 nm and about 1 μm, and betweenabout 100 nm and about 10 μm. Silicon particles of various sizes can beseparated by various methods such as by air classification, sieving orother screening methods. For example, a mesh size of 325 can be usedseparate particles that have a particle size less than about 44 μm fromparticles that have a particle size greater than about 44 μm.

Furthermore, the silicon particles may have a distribution of particlesizes. For example, at least about 90% of the particles may haveparticle size, for example a diameter or a largest dimension, betweenabout 10 nm and about 40 μm, between about 1 μm and about 15 μm, betweenabout 10 nm and about 1 μm, and/or larger than 200 nm.

In some embodiments, the silicon particles may have an average surfacearea per unit mass of between about 1 to about 100 m²/g, about 1 toabout 80 m²/g, about 1 to about 60 m²/g, about 1 to about 50 m²/g, about1 to about 30 m²/g, about 1 to about 10 m²/g, about 1 to about 5 m²/g,about 2 to about 4 m²/g, or less than about 5 m²/g.

In certain embodiments, the silicon particles are at least partiallycrystalline, substantially crystalline, and/or fully crystalline.Furthermore, the silicon particles may be substantially pure silicon.

Compared with the silicon particles used in conventional electrodes, thesilicon particles described herein for some embodiments can generallyhave a larger average particle size. In some embodiments, the averagesurface area of the silicon particles described herein can be generallysmaller. Without being bound to any particular theory, the lower surfacearea of the silicon particles described herein may contribute to theenhanced performance of electrochemical cells. Typical lithium ion typerechargeable battery anodes would contain nano-sized silicon particles.In an effort to further increase the capacity of the cell, smallersilicon particles (such as those in nano-size ranges) are being used formaking the electrode active materials. In some cases, the siliconparticles are milled to reduce the size of the particles. Sometimes themilling may result in roughened or scratched particle surface, whichalso increases the surface area. However, the increased surface area ofsilicon particles may actually contribute to increased degradation ofelectrolytes, which lead to increased irreversible capacity loss. FIGS.2A and 2B are SEM micrographs of an example embodiment of siliconparticles milled-down from larger silicon particles. As shown in thefigures, certain embodiments may have a roughened surface.

As described herein, certain embodiments include silicon particles withsurface roughness in nanometer-sized ranges, e.g., micron-sized siliconparticles with nanometer-sized features on the surface. FIGS. 2C and 2Dare SEM micrographs of an example embodiment of such silicon particles.Various such silicon particles can have an average particle size (e.g.,an average diameter or an average largest dimension) in the micron range(e.g., as described herein, between about 0.1 μm and about 30 μm) and asurface including nanometer-sized features (e.g., as described herein,between about 1 nm and about 1 μm, between about 1 nm and about 750 nm,between about 1 nm and about 500 nm, between about 1 nm and about 250nm, between about 1 nm and about 100 nm, between about 10 nm and about500 nm, between about 10 nm and about 250 nm, between about 10 nm andabout 100 nm, between about 10 nm and about 75 nm, or between about 10nm and about 50 nm). The features can include silicon.

Compared to the example embodiment shown in FIGS. 2A and 2B, siliconparticles with a combined micron/nanometer-sized geometry (e.g., FIGS.2C and 2D) can have a higher surface area than milled-down particles.Thus, the silicon particles to be used can be determined by the desiredapplication and specifications.

Even though certain embodiments of silicon particles havenanometer-sized features on the surface, the total surface area of theparticles can be more similar to micron-sized particles than tonanometer-sized particles. For example, micron-sized silicon particles(e.g., silicon milled-down from large particles) typically have anaverage surface area per unit mass of over about 0.5 m²/g and less thanabout 2 m²/g (for example, using Brunauer Emmet Teller (BET) particlesurface area measurements), while nanometer-sized silicon particlestypically have an average surface area per unit mass of above about 100m²/g and less than about 500 m²/g. Certain embodiments described hereincan have an average surface area per unit mass between about 1 m²/g andabout 30 m²/g, between about 1 m²/g and about 25 m²/g, between about 1m²/g and about 20 m²/g, between about 1 m²/g and about 10 m²/g, betweenabout 2 m²/g and about 30 m²/g, between about 2 m²/g and about 25 m²/g,between about 2 m²/g and about 20 m²/g, between about 2 m²/g and about10 m²/g, between about 3 m²/g and about 30 m²/g, between about 3 m²/gand about 25 m²/g, between about 3 m²/g and about 20 m²/g, between about3 m²/g and about 10 m²/g (e.g., between about 3 m²/g and about 6 m²/g),between about 5 m²/g and about 30 m²/g, between about 5 m²/g and about25 m²/g, between about 5 m²/g and about 20 m²/g, between about 5 m²/gand about 15 m²/g, or between about 5 m²/g and about 10 m²/g.

Various examples of micron-sized silicon particles with nanometer-sizedfeatures can be used to form certain embodiments of composite materialsas described herein. For example, FIG. 2E illustrates an example method200 of forming certain embodiments of the composite material. The method200 includes providing a plurality of silicon particles (for example,silicon particles having an average particle size between about 0.1 μmand about 30 μm and a surface including nanometer-sized features), block210. The method 200 further includes forming a mixture that includes aprecursor and the plurality of silicon particles, block 220. The method200 further includes pyrolysing the precursor, block 230, to convert theprecursor into one or more types of carbon phases to form the compositematerial.

With respect to block 210 of method 200, silicon with thecharacteristics described herein can be synthesized as a product orbyproduct of a Fluidized Bed Reactor (FBR) process. For example, in theFBR process, useful material can be grown on seed silicon material.Typically, particles can be removed by gravity from the reactor. Somefine particulate silicon material can exit the reactor from the top ofthe reactor or can be deposited on the walls of the reactor. Thematerial that exits the top of the reactor or is deposited on the wallsof the reactor (e.g., byproduct material) can have nanoscale features ona microscale particle. In some such processes, a gas (e.g., a nitrogencarrier gas) can be passed through the silicon material. For example,the silicon material can be a plurality of granular silicon. The gas canbe passed through the silicon material at high enough velocities tosuspend the solid silicon material and make it behave as a fluid. Theprocess can be performed under an inert atmosphere, e.g., under nitrogenor argon. In some embodiments, silane gas can also be used, for example,to allow for metal silicon growth on the surface of the siliconparticles. The growth process from a gas phase can give the siliconparticles the unique surface characteristics, e.g., nanometer-sizedfeatures. Since silicon usually cleaves in a smooth shape, e.g., likeglass, certain embodiments of silicon particles formed using the FBRprocess can advantageously acquire small features, e.g., innanometer-sized ranges, that may not be as easily achievable in someembodiments of silicon particles formed by milling from larger siliconparticles.

In addition, since the FBR process can be under an inert atmosphere,very high purity particles (for example, higher than 99.9999% purity)can be achieved. In some embodiments, purity of between about 99.9999%and about 99.999999% can be achieved. In some embodiments, the FBRprocess can be similar to that used in the production of solar-gradepolysilicon while using 85% less energy than the traditional Siemensmethod, where polysilicon can be formed as trichlorosilane decomposesand deposits additional silicon material on high-purity silicon rods at1150° C. Because nanometer-sized silicon particles have been shown toincrease cycle life performance in electrochemical cells, micron-sizedsilicon particles have not been contemplated for use as electrochemicalactive materials in electrochemical cells.

With respect to blocks 220 and 230 of method 200, forming a mixture thatincludes a precursor and the plurality of silicon particles, block 220,and pyrolysing the precursor, block 230, to convert the precursor intoone or more types of carbon phases to form the composite material can besimilar to blocks 101 and 105 respectively, of method 100 describedherein. In some embodiments, pyrolysing (e.g., at about 900° C. to about1350° C.) occurs at temperatures below the melting point of silicon(e.g., at about 1414° C.) without affecting the nanometer-sized featuresof the silicon particles.

In accordance with certain embodiments described herein, certainmicron-sized silicon particles with nanometer surface feature canachieve high energy density, and can be used in composite materialsand/or electrodes for use in electro-chemical cells to improveperformance during cell cycling.

Surface Modification of Silicon Particles

As described herein, silicon particles may be used as electrochemicallyactive material in an electrode. In some embodiments, the siliconparticles may include a surface coating. The surface coating may includesilicon carbide formed by a reaction between carbonized precursor and asilicon oxide surface layer on the silicon particles. In someembodiments, the surface coating comprising silicon carbide may act asan expansion buffer for silicon particles during cycling. Without beingbound by theory, the nature of the surface of the silicon particles(e.g., the nature of the oxide layer) may affect the silicon carbidelayer formed.

Silicon reacts with oxygen to create an oxide layer. For example,silicon surfaces exposed to air have a thin native oxide layer. Siliconalso can react with water (e.g., moisture treated silicon) to create anoxide layer. Oxide layers formed with oxygen can have differentproperties than oxide layers formed with water. Using moisture treatedsilicon particles in various embodiments of electrodes and/orelectrochemical cells described herein can provide surface coatingshaving silicon carbide with improved properties. In some instances, amore stable and/or a substantially stable solid electrolyteinterface/interphase (SEI) can be provided, improving capacity retentionand cycling performance.

FIG. 3A schematically illustrates an example moisture treated siliconparticle 300. For simplicity, the silicon particle 300 in FIG. 3A isillustrated as having a circular cross-sectional shape such as for aspherical or cylindrical particle. However, it would be understood that“silicon particle” 300 described herein can have any regular orirregular cross-sectional shape and is not limited to a spherical orcylindrical particle.

With continued reference to FIG. 3A, the example silicon particle 300comprises bulk material 301 and a surface 302. As described herein, thesilicon particle 300 can also have an outer region 303. For example, theouter region 303 can extend into the particle 300 some distance from thesurface 302. In some instances, the outer region 303 can be differentfrom the rest of the bulk material 301. For example, compared to therest of the bulk material 301, the outer region 303 can include a wetoxide as opposed to a dry oxide. As another example, the outer region303 can include one or more layers formed due to the presence ofmoisture (e.g., a hydroxide). In some embodiments, the outer region 303can be defined as extending about 25 nm, about 20 nm, or about 15 nmdeep from the surface. Other examples are possible.

In various embodiments, moisture treated silicon particles can beproduced by adding moisture to the silicon material that is used as aprecursor for the silicon particle 300. For example, adding moisture caninclude wetting and drying. In some embodiments, wetting can introduceliquid and/or an oxidizing agent to the silicon material. Liquids caninclude water, such as de-ionized water, distilled water, purifiedwater, ultrapure water, and/or alcohol, etc. Oxidizing agents caninclude potassium hydroxide, hydrogen peroxide, etc. Silicon materialthat can be used as a precursor include various types of siliconmaterials as described herein, including but not limited to siliconpowders, silicon fibers, porous silicon, ball-milled silicon, etc.

In some embodiments, silicon powders (or other silicon materials) can beimmersed into the liquid and/or oxidizing agent. The weight ratiobetween the silicon powders and liquid and/or oxidizing agent can be ina range from about 1:1 to about 1:6 or in a range within this range. Forexample, the weight ratio can be about 1:1, about 1:2, about 1:3, about1:4, about 1:5, about 1:6, etc. Other ratios are possible. The mixturecan be stirred to allow wetting of silicon surfaces. In some instances,the mixture can be stirred thoroughly or substantially thoroughly toallow the silicon surfaces to be fully wetted or substantially fullywetted. Drying can reduce the amount of (e.g., remove in some instances)the liquid and/or oxidizing agent from the silicon powders to form themoisture treated silicon particles 300.

In some embodiments, the silicon powders can be dried in air or an inertatmosphere. For example, the silicon powders can be dried at atemperature of at least about 140° C., at least about 145° C., at leastabout 150° C., at least about 155° C., at least about 160° C., at leastabout 165° C., at least about 170° C., at least about 175° C., at leastabout 180° C., etc. In some instances, the silicon powders can be driedat a temperature in the range of about 140° C. to about 180° C. or in arange within this range (e.g., about 145° C. to about 180° C., about150° C. to about 180° C., about 155° C. to about 180° C., about 155° C.to about 175° C., etc.). In some embodiments, the silicon powders can bedried at about 150° C., at about 155° C., at about 160° C., at about165° C., at about 170° C., at about 175° C., at about 180° C., etc. Thesilicon powders can subjected to the drying temperature until asufficient amount of the moisture has been removed. For example, thesilicon particles can be dried for at least about 20 hours, at leastabout 21 hours, at least about 22 hours, at least about 23 hours, atleast about 24 hours. In some instances, the silicon powders can bedried for an amount of time in a range from about 20 hours to about 28hours or in a range within this range (e.g., about 20 hours to about 27hours, about 22 hours to about 26 hours, etc.). In some embodiments, thesilicon powders are dried for about 21 hours, for about 22 hours, forabout 23 hours, for about 24 hours, for about 25 hours, for about 26hours, for about 27 hours, etc.

With continued reference to FIG. 3A, in addition to reducing and/orremoving the moisture from the silicon particles, drying can alsopromote conversion of an outer region 303 that is different from therest of the bulk material 301.

In some embodiments, the bulk material 301 of silicon particles caninclude aluminum. Silicon material (e.g., silicon powders) with morethan about 0.01%, more than about 0.02%, more than about 0.03%, morethan about 0.04%, more than about 0.05%, etc. of aluminum can havecertain characteristics and/or properties that change after moisturetreatment. Without being bound by theory, moisture treatment can allowthe aluminum in the bulk material 301 to be exposed more on the surface302 compared with no moisture treatment. For example, the surface 302may be modified such that more aluminum is exposed and/or more aluminumis dissolved and redistributed (e.g., aluminum redistributed toward thesurface). In some instances, more aluminum is exposed in the outerregions 303. In some instances, more aluminum is dissolved andredistributed in the outer regions 303 while some of the aluminum mayremain within the bulk material 301 surrounded by the outer regions 303.In some other instances, all or substantially all of the aluminum may beexposed and/or dissolved and redistributed in the outer regions 303 ofthe silicon particles 300. Accordingly, in various embodiments, theouter region 303 of moisture treated silicon particles 300 can includemore aluminum compared with the outer region of untreated siliconparticles. Without being bound by theory, when used in compositematerials described herein, the aluminum in the outer regions 303 of themoisture treated silicon particles 300 can affect the silicon carbidelayer formed in composite materials, e.g., producing a more stable SEIlayer.

In some embodiments, the outer regions 303 of the moisture treatedsilicon particles 300 can include an amount of aluminum such that a bulkmeasurement of the aluminum comprises at least about 0.01% by weight ofthe silicon particles 300. For example, the silicon particles caninclude a bulk measurement of aluminum of at least about 0.01%, at leastabout 0.02%, at least about 0.03%, at least about 0.04%, at least about0.05%, at least about 0.06%, at least about 0.07%, at least about 0.08%,at least about 0.09%, at least about 0.1%, etc. In some embodiments, thebulk measurement of aluminum can be in a range from about 0.01% to about1%, from about 0.02% to about 1%, from about 0.03% to about 1%, fromabout 0.04% to about 1%, from about 0.05% to about 1%, from about 0.06%to about 1%, from about 0.07% to about 1%, from about 0.08% to about 1%,from about 0.09% to about 1%, from about 0.1% to about 0.6%, from about0.1% to about 0.7%, from about 0.1% to about 0.8%, from about 0.1% toabout 0.9%, from about 0.1% to about 1%, from about 0.2% to about 0.6%,from about 0.2% to about 0.7%, from about 0.2% to about 0.8%, from about0.2% to about 0.9%, from about 0.2% to about 1%, from about 0.3% toabout 0.6%, from about 0.3% to about 0.7%, from about 0.3% to about0.8%, from about 0.3% to about 0.9%, from about 0.3% to about 1%, etc.by weight of the silicon particles. Other examples are possible.

In some embodiments, the bulk measurement of the aluminum can bemeasured by X-ray Photoelectron Spectroscopy (XPS). In XPS, theelemental composition of a sample can be measured by irradiating x-raysand measuring the kinetic energy and number of electrons escaping thesample. The measurement may be surface-sensitive and hence measure theamount of aluminum present at least at the surface of the siliconparticles (e.g., at least in the outer regions 303). Depending on themeasurement sensitivity, in some instances, the bulk measurement ofaluminum can provide the amount of aluminum in the outer regions 303 ofthe silicon particles 300 or the amount of aluminum in the entirematerial of the silicon particles 300 (e.g., the amount in the outerregions 303 and in other areas within the bulk material 301). Othermethods known in the art or yet to be developed can be used to measurethe amount of aluminum. For example, in some instances, InductivelyCoupled Plasma (ICP) analysis including Inductively Coupled PlasmaAtomic Emission Spectroscopy (ICP-AES) or Inductively Coupled PlasmaMass Spectrometry (ICP-MS) can be used to measure the amount ofaluminum. As another example, in some instances, Energy Dispersive X-raySpectroscopy (EDS) can be used to measure the amount of aluminum. Asanother example, in some instances, X-ray Fluorescence (XRF) can be usedto measure the amount of aluminum.

In some embodiments, the outer regions 303 of the silicon particles 300can include an amount of aluminum of at least about 0.001% by weight ofthe silicon particles. For example, the silicon particles 300 caninclude an amount of aluminum in the outer regions 303 of at least about0.001%, at least about 0.002%, at least about 0.003%, at least about0.004%, at least about 0.005%, etc. In some embodiments, the amount ofaluminum in the outer regions 303 can be in a range from about 0.001% toabout 0.01%, from about 0.002% to about 0.01%, from about 0.003% toabout 0.01%, from about 0.004% to about 0.01%, from about 0.005% toabout 0.01%, etc. by weight of the silicon particles.

In some instances, the outer regions 303 of the silicon particles 300can include an amount of aluminum of at least about 0.01%, at leastabout 0.02%, at least about 0.03%, at least about 0.04%, at least about0.05%, at least about 0.06%, at least about 0.07%, at least about 0.08%,at least about 0.09%, at least about 0.1%, etc. In some embodiments, theamount of aluminum in the outer regions 303 can be in a range from about0.01% to about 1%, from about 0.02% to about 1%, from about 0.03% toabout 1%, from about 0.04% to about 1%, from about 0.05% to about 1%,from about 0.06% to about 1%, from about 0.07% to about 1%, from about0.08% to about 1%, from about 0.09% to about 1%, from about 0.1% toabout 0.6%, from about 0.1% to about 0.7%, from about 0.1% to about0.8%, from about 0.1% to about 0.9%, from about 0.1% to about 1%, fromabout 0.2% to about 0.6%, from about 0.2% to about 0.7%, from about 0.2%to about 0.8%, from about 0.2% to about 0.9%, from about 0.2% to about1%, from about 0.3% to about 0.6%, from about 0.3% to about 0.7%, fromabout 0.3% to about 0.8%, from about 0.3% to about 0.9%, from about 0.3%to about 1%, etc. by weight of the silicon particles. Other examples arepossible.

In various embodiments, the aluminum in the outer regions 303 of thesilicon particles 300 can be include aluminum oxide, aluminum silicide,or a combination of aluminum oxide and aluminum silicide.

In some embodiments, the moisture treated silicon particles 300 can havesurfaces 302 that provide a lower contact angle than those provided bysurfaces of untreated silicon particles. Some embodiments of moisturetreated silicon particles 300 can have an average contact angle lessthan about 87.3°, less than about 87.2°, less than about 87.1°, lessthan about 87°, less than about 86.9°, less than about 86.8°, less thanabout 86.7°, less than about 86.6°, less than about 86.5°, etc. Forexample, some moisture treated silicon particles can have an averagecontact angle in a range from about 80° to about 87.2° or in a rangetherebetween (e.g., from about 80° to about 87.1°, from about 80° toabout 87°, from about 80° to about 86.9°, from about 81° to about 87.1°,from about 81° to about 87°, from about 81° to about 86.9°, from about82° to about 87.1°, from about 82° to about 87°, from about 82° to about86.9°, etc.). Without being bound by theory, by having a lower contactangle than those provided by surfaces of untreated silicon particles,various embodiments of moisture treated silicon particles 300 can havesurfaces with higher wettability allowing increased mixability inslurries, e.g., in mixtures that form various embodiments of compositematerials described herein.

For example, various examples of moisture treated silicon particles canbe used to form certain embodiments of composite materials as describedherein. For example, the composite material can be used in an electrodefor use in an electrochemical cell. The moisture treated siliconparticles can have certain characteristics and/or properties asdescribed herein that may improve capacity retention and cyclingperformance. Without being bound by theory, moisture treated siliconparticles have surfaces that can provide for a more stable and/orsubstantially stable SEI.

Certain embodiments can include a composite material film. The compositematerial film can include any of the examples of composite materialfilms described herein. For example, the composite material film canhave greater than 0% and less than about 99% by weight of siliconparticles (e.g., about 50% to about 99% by weight, about 60% to about99% by weight, about 70% to about 99% by weight, about 75% to about 99%by weight, about 80% to about 99% by weight, etc.), or greater than 0%and less than about 95% by weight of silicon particles (e.g., about 50%to about 95% by weight, about 60% to about 95% by weight, about 70% toabout 95% by weight, about 75% to about 95% by weight, about 80% toabout 95% by weight, etc.). The silicon particles can include moisturetreated silicon particles and/or a combination of moisture treatedsilicon particles and other silicon particles described herein (e.g.,silicon particles with an average particle size from about 0.1 μm toabout 40 μm, silicon particles with an average particle size from about1 μm to about 20 μm, micron-sized silicon particles with nanometer-sizedfeatures, etc.). The silicon particles can include aluminum as describedherein. The silicon particles can be from about 90% to about 99% puresilicon. As described herein, the silicon particles can have surfacecoatings comprising silicon carbide or a mixture of carbon and siliconcarbide as shown in FIG. 1B. The surface coatings can be a substantiallycontinuous layer. In some embodiments, the surface coatings can includesilicon monoxide (SiO), silicon dioxide (SiO₂), and/or silicon oxide(SiO_(x)). The composite material film can also have greater than 0% andless than 90% by weight of one or more types of carbon phases. The oneor more types of carbon phases can be a substantially continuous phase.In some embodiments, the one or more types of carbon phases can beelectrochemically active and/or electrically conductive. In variousembodiments, the composite material film can be self-supported.

FIG. 3B illustrates an example method 400 of forming certain embodimentsof the composite material. The method 400 can include providing amixture comprising a carbon precursor and silicon particles (e.g.,moisture treated silicon particles), block 410. The method 400 can alsoinclude pyrolysing the precursor to convert the precursor into one ormore types of carbon phases, block 420. The method 400 can furtherinclude forming silicon carbide on surfaces of the silicon particles,block 430.

With respect to block 410 of method 400, the silicon particles havingthe characteristics and/or properties as described herein can beprovided by using moisture treated silicon particles. In someembodiments, the moisture treated silicon particles can include siliconmaterial treated with a liquid and/or moisture in one or more of avariety of ways. The moisture treated silicon particles can be providedby wetting and drying as described herein. For example, the siliconmaterial can be processed in liquid. In some embodiments, the liquid caninclude water (e.g., de-ionized water, distilled water, purified water,ultrapure water, etc.), alcohol, and/or other oxidizing agent such aspotassium hydroxide or hydrogen peroxide. Silicon material can be boiledin liquid to produce liquid-boiled silicon particles, decanted in liquidto produce liquid-decanted silicon particles, and/or steamed with liquidto produce liquid-steamed silicon particles.

As described herein, the silicon particles can be added to a mixturecomprising a carbon precursor. In some such embodiments, alternativelyor additionally to treating the silicon material alone with moisture,the silicon particles can be treated with moisture present in and/oradded to the carbon precursor. For example, as described herein, thecarbon precursor can include a polymer and a solvent. Moisture can bepresent in and/or added to the polymer and/or solvent. The polymer andsolvent can include any of the polymers and solvents described herein.As some examples, the carbon precursor can include polyamideimide,polyamic acid, polyimide, phenolic resin, epoxy resin, etc. The solventcan comprise N-methyl-pyrrolidone (NMP), acetone, diethyl ether, gammabutyrolactone, isopropanol, dimethyl carbonate, ethyl carbonate,dimethoxyethane, ethanol, methanol, etc. Moisture can be added to thepolymer and/or solvent. In some instances, the carbon precursor caninclude a water-soluble polymer, and the solvent can include water.

The silicon material alone or in combination with the mixture can beheat treated to dry the particles. The wetting and drying of the siliconparticles can be performed sequentially. However, in some embodiments,the wetting and drying of the silicon particles can be performedsimultaneously. For example, moisture can be added to a drying/heatingchamber and silicon material alone or in combination with the mixturecan be placed into the heating chamber. The heat treatment can beperformed at a temperature and time as described herein.

As described herein, the method 400 can also include various steps suchas casting the mixture on a substrate, drying the mixture to form afilm, removing the film from the substrate, and placing the film in ahot press. Moisture can be added to the mixture or film in any of thesesteps. Moisture can be added to the mixture or film alternatively oradditionally to adding moisture to the silicon material alone.

With respect to block 420 of method 400, the carbon precursor can bepyrolyzed as described herein to convert the precursor into one or moretypes of carbon phases. In some embodiments, moisture can be added inthe pyrolysis chamber. Moisture can be added in the pyrolysis chamberalternatively or additionally to adding moisture to the silicon materialalone, alternatively or additionally to adding moisture to the mixtureor film, and/or alternatively or additionally to adding moisture intothe drying chamber.

With respect to block 430, silicon carbide can be formed on surfaces ofthe silicon particles. Using silicon particles with certain surfacecharacteristics and properties as described herein (e.g., using moisturetreated silicon particles) can affect the silicon carbide layer formedand form a more stable SEI.

As described herein, moisture treated silicon particles can have certaincharacteristics and properties beneficial in producing compositematerials in electrodes for use in electrochemical cells. It would beunderstood that it may be possible to provide silicon particles havingthe desired characteristics and properties described herein withoutmoisture treatment. Some such non-moisture treated silicon particles canalso be used in composite materials and/or methods of forming compositematerials.

EXAMPLES

The below example processes for anode fabrication generally includemixing components together, casting those components onto a removablesubstrate, drying, curing, removing the substrate, then pyrolyzing theresulting samples. N-Methyl-2-pyrrolidone (NMP) was typically used as asolvent to modify the viscosity of any mixture and render it castableusing a doctor blade approach.

Example 1

In Example 1, a polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), conductive carbonparticles (Super P from Timcal corp.), and silicon particles (from AlfaAesar corp.) were mixed together for 5 minutes using a Spex 8000Dmachine in the weight ratio of 200:55:5:20. The mixture was then castonto aluminum foil and allowed to dry in a 90° C. oven, to drive awaysolvents, e.g., NMP. This is followed by a curing step at 200° C. in ahot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed around an hour at 1175° C. under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 57.9% ofgraphite particles, 5.3% of carbon resulting from Super P, and 21.1% ofsilicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 4.

Example 2

In Example 2, silicon particles (from EVNANO Advanced Chemical MaterialsCo. Ltd.) were initially mixed with NMP using a Turbula mixer for aduration of one hour at a 1:9 weight ratio. Polyimide liquid precursor(PI 2611 from HD Microsystems corp.), graphite particles (SLP30 fromTimcal corp.), and carbon nanofibers (CNF from Pyrograf corp.) were thenadded to the Si:NMP mixture in the weight ratio of 200:55:5:200 andvortexed for around 2 minutes. The mixture was then cast onto aluminumfoil that was covered by a 21 μm thick copper mesh. The samples werethen allowed to dry in a 90° C. oven to drive away solvents, e.g., NMP.This was followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed for around an hourat 1000° C. under argon. The process resulted in a composition of 15.8%of PI 2611 derived carbon, 57.9% of graphite particles, 5.3% of CNF, and21.1% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 5.

Example 3

In Example 3, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and 325 mesh silicon particles (from Alfa Aesar corp.) weremixed together using a Turbula mixer for a duration of 1 hour in theweight ratios of 40:1. The mixture was then cast onto aluminum foil andallowed to dry in a 90° C. oven to drive away solvents, e.g., NMP. Thiswas followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed around an hour at1175° C. under argon flow. The process resulted in a composition of 75%of PI 2611 derived carbon and 25% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 6.

Example 4

In Example 4, silicon microparticles (from Alfa Aesar corp.), polyimideliquid precursor (PI 2611 from HD Microsystems corp.), graphiteparticles (SLP30 from Timcal corp.), milled carbon fibers (from FibreGlast Developments corp.), carbon nanofibers (CNF from Pyrograf corp.),carbon nanotubes (from CNANO Technology Limited), conductive carbonparticles (Super P from Timcal corp.), conductive graphite particles(KS6 from Timca corp.) were mixed in the weight ratio of20:200:30:8:4:2:1:15 using a vortexer for 5 minutes. The mixture wasthen cast onto aluminum foil. The samples were then allowed to dry in a90° C. oven to drive away solvents, e.g., NMP. This was followed by acuring step at 200° C. in a hot press, under negligible pressure, for atleast 12 hours. The aluminum foil backing was then removed by etching ina 12.5% HCl solution. The remaining film was then rinsed in DI water,dried and then pyrolyzed for around an hour at 1175° C. under argon. Theprocess resulted in a composition similar to the original mixture butwith a PI 2611 derived carbon portion that was 7.5% the original weightof the polyimide precursor.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 7.

Example 5

In Example 5, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and silicon microparticles (from Alfa Aesar corp.) were mixedtogether using a Turbula mixer for a duration of 1 hours in the weightratio of 4:1. The mixture was then cast onto aluminum foil covered witha carbon veil (from Fibre Glast Developments Corporation) and allowed todry in a 90° C. oven to drive away solvents, e.g., NMP. This wasfollowed by a curing step at 200° C. in a hot press, under negligiblepressure, for at least 12 hours. The aluminum foil backing was thenremoved by etching in a 12.5% HCl solution. The remaining film was thenrinsed in DI water, dried and then pyrolyzed around an hour at 1175° C.under argon flow. The process resulted in a composition of approximately23% of PI 2611 derived carbon, 76% of silicon by weight, and the weightof the veil being negligible.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium nickel manganese cobalt oxide (NMC) cathode. A typicalcycling graph is shown in FIG. 8.

Example 6

In Example 6, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), and siliconmicroparticles (from Alfa Aesar corp.) were mixed together for 5 minutesusing a Spex 8000D machine in the weight ratio of 200:10:70. The mixturewas then cast onto aluminum foil and allowed to dry in a 90° C. oven, todrive away solvents (e.g., NMP). The dried mixture was cured at 200° C.in a hot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed at 1175° C. for about one hour under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 10.5% ofgraphite particles, 73.7% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. The anodes where charged to 600mAh/g each cycle and the discharge capacity per cycle was recorded. Atypical cycling graph is shown in FIG. 9.

Example 7

In Example 7, PVDF and silicon particles (from EVNANO Advanced ChemicalMaterials Co), conductive carbon particles (Super P from Timcal corp.),conductive graphite particles (KS6 from Timcal corp.), graphiteparticles (SLP30 from Timcal corp.) and NMP were mixed in the weightratio of 5:20:1:4:70:95. The mixture was then cast on a copper substrateand then placed in a 90° C. oven to drive away solvents, e.g., NMP. Theresulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 10.

Example 8

Multiple experiments were conducted in order to find the effects ofvarying the percentage of polyimide derive carbon (e.g. 2611 c) whiledecreasing the percentage of graphite particles (SLP30 from Timcalcorp.) and keeping the percentage of silicon microparticles (from AlfaAesar corp.) at 20 wt. %.

As shown in FIGS. 11A and 11B, the results show that more graphite andless 2611 c was beneficial to cell performance by increasing thespecific capacity while decreasing the irreversible capacity. Minimizing2611 c adversely affected the strength of the resultant anode so a valueclose to 20 wt. % can be preferable as a compromise in one embodiment.

Example 9

Similar to example 8, if 2611 c is kept at 20 wt. % and Si percentage isincreased at the expense of graphite particles, the first cycledischarge capacity of the resulting electrode is increased. FIG. 12shows that a higher silicon content can make a better performing anode.

Example 10

When 1 mil thick sheets of polyimide are pyrolized and tested inaccordance with the procedure in Example 1. The reversible capacity andirreversible capacity were plotted as a function of the pyrolysistemperature. FIG. 13 indicates that, in one embodiment, it is preferableto pyrolyze polyimide sheets (Upilex by UBE corp) at around 1175° C.

Additional Examples

FIG. 14 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer. The composite anode film has a thickness ofabout 30 microns and has a composition of about 15.8% of PI 2611 derivedcarbon, about 10.5% of graphite particles, and about 73.7% of silicon byweight.

FIGS. 15-20 are scanning electron microscope (SEM) micrographs of acomposite anode film. The compositions of the composite anode film wereabout 15.8% of PI 2611 derived carbon, about 10.5% of graphiteparticles, and about 73.7% of silicon by weight. FIGS. 15 and 16 showbefore being cycled (the out-of-focus portion is a bottom portion of theanode and the portion that is in focus is a cleaved edge of thecomposite film). FIGS. 17, 18, and 19 are SEM micrographs of a compositeanode film after being cycled 10 cycles, 10 cycles, and 300 cycles,respectively. The SEM micrographs show that there is not any significantpulverization of the silicon and that the anodes do not have anexcessive layer of solid electrolyte interface/interphase (SEI) built ontop of them after cycling. FIG. 20 are SEM micrographs of cross-sectionsof composite anode films.

Described below are measured properties of example silicon particles.These examples are discussed for illustrative purposes and should not beconstrued to limit the scope of the disclosed embodiments.

FIG. 21 is an x-ray powder diffraction (XRD) graph of the sample siliconparticles. The XRD graph suggests that the sample silicon particles weresubstantially crystalline or polycrystalline in nature.

FIGS. 22-25 are scanning electron microscope (SEM) micrographs of thesample silicon particles. Although the SEM micrographs appear to showthat the silicon particles may have an average particle size greaterthan the measured average particle size of about 300 nm, without beingbound by theory, the particles are believed to have conglomeratedtogether to appear to be larger particles.

FIG. 26 is a chemical analysis of the sample silicon particles. Thechemical analysis suggests that the silicon particles were substantiallypure silicon.

FIGS. 27A and 27B are example particle size histograms of twomicron-sized silicon particles with nanometer-sized features. Theparticles were prepared from a FBR process. Example silicon particlescan have a particle size distribution. For example, at least 90% of theparticles may have a particle size, for example, a diameter or a largestdimension, between about 5 μm and about 20 μm (e.g., between about 6 μmand about 19 μm). At least about 50% of the particles may have aparticle size between about 1 μm and about 10 μm (e.g., about 2 μm andabout 9 μm). Furthermore, at least about 10% of the particles may have aparticle size between about 0.5 μm and about 2 μm (e.g., about 0.9 μmand about 1.1 μm).

FIG. 28 is a plot of discharge capacity during cell cycling comparingtwo types of example silicon particles. The performance of four samplesof silicon particles (micron-sized particles with nanometer-sizedfeatures) prepared by the FBR process are compared with five samples ofsilicon particles prepared by milling-down larger silicon particles.Thus, certain embodiments of silicon particles with the combinedmicron/nanometer geometry (e.g., prepared by the FBR process) can haveenhanced performance over various other embodiments of silicon particles(e.g., micron-sized silicon particles prepared by milling down fromlarger particles). The type of silicon particles to use can be tailoredfor the intended or desired application and specifications.

Examples of Moisture Treated Silicon Particles

Silicon powders were immersed into de-ionized water with a 1:4 weightratio. The mixture was stirred thoroughly and dried at 160° C. for 24hours. FIG. 29 shows X-ray Photoelectron Spectroscopy (XPS) spectra ofthe aluminum 2p peak in example moisture treated silicon particlescompared with untreated silicon particles. As shown in FIG. 29, moisturetreated silicon particles have a characteristic aluminum peak,indicating presence of aluminum at the surface. Aluminum oxide and/oraluminum silicide were present in quantities greater than 0.001% (e.g.,greater than 0.01%, greater than 0.1%, etc.) in the outer regions (e.g.,about 20 nm depth) of the silicon particles. Without being bound bytheory, moisture treatment can allow more aluminum at the surface.

Table I includes contact angle (CA) measurements for the differentsamples of the example moisture treated silicon particles. The contactangles (CA) were measured by the Washburn method using a forcetensiometer.

TABLE I Sample 1 Sample 2 CA - Measurement 1 87.072 84.821 CA -Measurement 2 82.909 82.646 CA - Measurement 3 84.294 85.104 CA -Average 84.758 84.190 CA - Standard Deviation 2.120 1.345

As shown in Table 1, the average contact angle for the samples was84.19° and 84.76°. The contact angles measured before moisture treatmentwere higher (e.g., greater than 87.2°) than after treatment.

The samples described herein were mixed with graphite, NMP, and resin.The mixture was formed into an anode precursor (e.g., coated on asubstrate, dried, and removed from the substrate), and pyrolysed to forma self-supporting anode film as described herein.

The sample anodes were built against lithium cobalt oxide (LCO)-basedcathodes to form a battery. FIG. 30 shows a graph of the capacityretention versus cycle number for the sample batteries. As shown in FIG.30, the capacity retention of the battery with moisture treated siliconparticles improved by 30% at 400 cycles compared to the battery withuntreated silicon particles.

As described herein, moisture treatment of silicon particles can providea relatively simple and practical method to control the SEI formation inelectrodes and to significantly improve electrochemical cellperformance. Moisture treatment can also provide a method to reducecell-to-cell variation in performance, which can allow the product to bemore stable and less dependent on process variations. Further, moisturetreatment of silicon particles can provide a method to control theinitial lithiation level in the active particles, which can allow a moreeven lithiation and stress and reduce localized and concentratedreactions that may accelerate fading. Moreover, although variousembodiments described herein include silicon particles for use inlithium-ion batteries, the silicon particles can be used in otherbattery systems involving silicon-metal alloying processes, e.g., sodiumor magnesium systems.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. Silicon particles for use in an electrode in anelectrochemical cell, the silicon particles comprising: surfacesproviding an average contact angle less than about 87.2°; and outerregions extending about 20 nm deep from the surfaces, the outer regionscomprising an amount of aluminum such that a bulk measurement of thealuminum comprises at least about 0.01% by weight of the siliconparticles.
 2. The silicon particles of claim 1, wherein the bulkmeasurement of the aluminum comprises from about 0.01% to about 1% byweight of the silicon particles.
 3. The silicon particles of claim 1,wherein the bulk measurement of the aluminum comprises at least about0.05% by weight of the silicon particles.
 4. The silicon particles ofclaim 3, wherein the bulk measurement of the aluminum comprises fromabout 0.05% to about 1% by weight of the silicon particles.
 5. Thesilicon particles of claim 1, wherein the bulk measurement of thealuminum comprises at least about 0.1% by weight of the siliconparticles.
 6. The silicon particles of claim 5, wherein the bulkmeasurement of the aluminum comprises from about 0.1% to about 1% byweight of the silicon particles.
 7. The silicon particles of claim 6,wherein the bulk measurement of the aluminum comprises from about 0.1%to about 0.6% by weight of the silicon particles.
 8. The siliconparticles of claim 1, wherein the outer regions comprise aluminum oxide.9. The silicon particles of claim 1, wherein the outer regions comprisealuminum silicide.
 10. The silicon particles of claim 1, wherein theaverage contact angle is from about 82° to about 87.1°.
 11. An electrodefor use in an electrochemical cell comprising the silicon particles ofany of claim
 1. 12.-63. (canceled)